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Chemistry Chapter 4 Vocabulary
atom – Smallest particle capable of chemical reactions.
law of definite composition – A law stating that every compound is formed of elements combined in
ratios of mass that are unique for that compound.
cathode rays – The stream of electrons emitted from the cathode in a cathode-ray tube.
electron – A particle with a -1 charge and a mass of 0.000055 u founding orbiting the nucleus in an
atom.
alpha particle – The nucleus of the helium atom (two protons, two neutrons); represented by He2+ or
He-42+ .
nucleus – The dense central part of an atom made up of protons and neutrons. The nucleus contains
virtually all of the atom’s mass but only a small part of the atomic volume.
proton – A particle in the nucleus of an atom with a +1 charge and a mass of 1.0073 .
atomic number – The number of protons in the nucleus in the nucleus of an atom.
neutron – A neutral particle within the nucleus; has a mass of 1.0087, slightly greater than the mass of
a proton.
continuous spectrum – A complete visual spectrum with no frequency gaps; emitted only by an ideal
luminous object.
line spectrum – A spectrum showing only certain colors or wavelengths of light.
principal energy level – A region around the nucleus containing a specified group of electrons in
sublevels and orbitals.
quantized – A term describing something that has separate, discrete values.
ground state – An atom’s lowest energy state, containing as many as seven levels.
photon – A packet of electromagnetic energy; an elementary nonmaterial “particle” that transmits the
electromagnetic force in the standard model of matter.
de Broglie’s hypothesis – The concept that if waves can behave like particles, the particles can behave
like waves.
Heisenberg uncertainty principle – The law stating that it is impossible to know both the energy or
momentum and the exact position of an electron at the same time.
orbital – A four-dimensional region of space in which as many as two electrons may exist; sections of
the sublevels.
principal quantum number – The number indicating the main or principal energy level in which an
electron is found.
quantum numbers – Four numbers that describe the location of an electron in an atom; the first
identifies the relative size of the principal energy level, the second describes the type of sublevel; the
third indicates the direction of the orbital in space, and the fourth describes the spin of the electron.
azimuthal quantum number – The number that defines the symmetric shapes arranged around the
nucleus within which electrons may be found; sometimes called the sublevel quantum number.
magnetic quantum number – The number that defines the spatial orientation of the orbitals within the
sublevel; sometimes called the orbital quantum number.
electron spin quantum number – The number that describes the two possible “spin” states of a pair of
electrons in an orbital.
Pauli exclusion principle – The rule that an orbital can hold only two electrons with the opposite spin.
Therefore, no two electrons in the same atom can have the same set of four quantum numbers.
sublevel – A portion of the principal energy level made up of one or more orbitals.
Aufbau principle – The principle stating that the electron configuration of an atom mat be obtained by
building on the electron configuration of an atom of the lower atomic number; the electrons fill the
sublevels in the order given by the diagonal rule.
electron configuration – A representation of how electrons are positioned in an atom; a number
indicates the principle energy level, a letter indicates the principal energy level, a letter indicates the
sublevel, and a subscript denotes the number of electrons contained within the sublevel.
orbital notation – A diagrammatic representation that uses dashes and arrows to show the principal
energy levels, sublevels, and orbitals for all electrons in an atom.
Hund’s rule – The rule stating that as electrons fill a sublevel, all orbitals receive one electron before
any receives two.
isotope – one of two or more atoms of the same element with the same number of protons (atomic
number) but different numbers of neutrons.
mass number – a number equal to the sum of the number of protons and neutrons in the nucleus of an
atom.
isotopic notation – A naming convention that includes the symbol, atomic number, and mass number
of an element.
unified atomic mass unit – A small unit of mass equal to on-twelfth of the mass of the nucleus of a
carbon-12 atom, or approximately the same size as a proton or neutron. This unit is used to represent
the mass of atoms and molecules.
valence electron – The most loosely bound electrons, which are usually found in the highest or
outermost energy level of a neutral atom.
electron-dot notation – A chemical symbol with surrounding dots representing the valence electrons of
an atom.
cation – A positively charged ion.
anion – A negatively charged ion.